Sidetone is the reproduction of acoustic signals in a telephone receiver corresponding to voice signals generated by the transmitter of the same telephone set. The sidetone signal results from a portion of the transmitted signal being coupled to the receiver by way of a four-to-two wire hybrid circuit which is a differential balance circuit that permits voice signals to be transmitted and received via amplifying circuits connected to the same telephone line without in any way adversely affecting one another.
The presence of some sidetone is considered desirable as an indication of a properly functioning set and indeed produces the expected normal condition of hearing one's own voice when speaking. An adverse condition may develop, however, should the sidetone level become excessive. Such condition degrades the transmission quality by inherently causing a talker to reduce speech levels. For a listener in a noisy environment, high sidetone levels result in reduced signal-to-noise ratios that mask incoming receive signals by acoustic noise coupled through the transmitter.
Sidetone level in a telephone set is determined by five operational parameters:
1. transmit sensitivity; PA1 2. receive sensitivity; PA1 3. impedance of the telephone set, i.e., the impedance measured looking into the line terminals of the telephone set; PA1 4. impedance for minimum sidetone, i.e., the impedance connected to the line terminals of the telephone set which causes the sidetone to completely vanish; and PA1 5. near end input impedance of the telephone connection, i.e., the impedance measured looking into the telephone line from the telephone set.
The first four parameters represent variables coming under discretionary control of a telephone set designer. Of these parameters, only the fourth may be varied freely to control sidetone performance of the telephone set since the first three parameters must normally be chosen to achieve satisfactory voice transmission performance in the system.
The fifth parameter is highly variable and depends upon the topology of a telephone connection which is influenced by the length and gauge of telephone cables, the presence of loading coils and their spacing, the presence of bridge-taps and the far end termination of the telephone line.
The variable nature of telephone connections, which can be unpredictable, places reliance on the fourth parameter which is a controllable factor through which minimum sidetone may be obtained so as to provide the best possible sidetone performance over an expected wide range of telephone connections.